A general spark plug includes a metal shell, a center electrode, and an insulator. Known shapes of the insulator include one that has, sequentially from a rear end side, a first cylindrical portion, a truncated cone-shaped portion, and a second cylindrical portion whose outer diameter is smaller than that of the first cylindrical portion. The first cylindrical portion is a cylindrical part formed inside the metal shell. The truncated cone-shaped portion is a part that is formed on a front end side of the first cylindrical portion and whose outer diameter becomes smaller toward the front end side. The second cylindrical portion is a part that is formed on the front end side of the truncated cone-shaped portion and whose at least one portion projects out from a front end surface of the metal shell. The first cylindrical portion, the truncated cone-shaped portion, and the second cylindrical portion are all hollow, and a center electrode is disposed in the hallow space (e.g., Japanese Patent Application Laid-Open (kokai) No. 2005-183177).
On the other hand, in recent years, there is a trend to increase the compression ratio of an engine, and the voltage (required voltage) for discharging at a regular discharge position (gap) has been increased in a spark plug. When the required voltage is high, voltage resistance is demanded strictly, and side sparking (discharge between the insulator and the metal shell) occurs easily. Side sparking occurs easily particularly around the front end surface of the metal shell.
Reducing the diameter of the center electrode is effective for improving anti-side sparking characteristic and voltage resistance without increasing the overall size of the spark plug. However, since the heat capacity of the center electrode becomes smaller when the diameter of the center electrode becomes smaller, the temperature of the center electrode rises easily, and oxidation of the center electrode is accelerated. Thus, reducing the diameter of the center electrode has been conventionally difficult.
Another method for suppressing side sparking is to radially separate, at around the front end surface of the metal shell, the outer circumference of the insulator from the inner circumference of the metal shell as much as possible. With this method, reducing the outer diameter of the insulator can be achieved.
However, an attempt to ensure certain thickness of the insulator while reducing the outer diameter of the insulator results in thinning of the center electrode disposed inside the insulator and causes the above described problem. On the other hand, when the insulator is thinned, the heat capacity of the insulator reduces, and the temperature of the center electrode easily rises. As a result, oxidation of the center electrode is accelerated.
Since the above described dilemma has existed conventionally, simultaneously achieving improvement in voltage resistance, suppression of oxidation of the center electrode, and suppression of side sparking has been difficult.
The present invention is intended to solve the above described problem, and can be embodied in the following modes.